The Stars Will Disappear Forever
Our view of the universe has an expiration date.
What are Stars actually?
We look at the sky at night and see all the countless twinkling stars but most of us do not know that they are actually Suns of another solar systems which far far away from us. The stars that is nearest to us is what we call the Sun. These stars are basically giant spheres of gas and plasma held together by the insanely strong gravity (strength of the gravity depends on Mass and Distance!). We can fit around 1,300,000 Earths inside the Sun so you can guess how gigantic the Sun is and that explains its insanely strong gravity, that not just pulls its own core together but also the entire solar system exists because of our dear Sun. There are a lot of other interesting facts about the Sun or stars, but let's not get lost into the universe just yet!
Apologies for the cognitive overload but I couldn't hold myself back to drop in another mind boggling fact. There are around 400 billion stars in our galaxy, but the number becomes crazy when we look at the entire observable universe, which boasts around 1 septillion stars, which is 1,000,000,000,000,000,000,000,000 stars!
The Journey of Light
The light from the Sun takes 8 minutes and 20 seconds to reach Earth. So if you are outside and you are feeling the warmth of the sunlight, that exact light particle (or photon) started its journey around 8 mins ago, and finally reached you! Let's do some basic math to understand this. If you remember the classic speed, distance and time formula from the school
Let me fill in the data you need to calculate the time for the light from the Sun to reach Earth. The Sun is around 150 million kilometers away from us and the speed of light (the fastest thing in the entire universe!) is roughly 300,000 kilometers per SECOND!
That's about 8 minutes 20 second for ya! If you think this is a surprising, hold on... let me remind you that the sun is the closest star to us. The second closest star is Proxima Centauri which is 4.24 light years away from us or roughly 40 trillion kilometers away. A light year is basically the distance travelled by light (the absolute fastest thing in the universe) in a year, which is around 9.46 trillion kilometers, or thanks to google exactly 9,460,730,472,580 kilometers. The light from this star takes 4 years and 3 months to reach us so when astronomers are looking through their telescope, they are seeing the light which left the star 4 years ago and reached us now.
Now for a second, assume there exists an advanced Alien civilization far away from us in some galaxy, let's say 66 million light years away from Earth. They would not see us the way we are today, but a version of earth that is 66 million years old, so they would see dinosaurs! I hope now you understand the vastness of the space, and how light works and are curious why we would loose the stars we see today.
Expansion of the Space
As you read these lines, space, THE space, is expanding. I mean literally increasing in the size every second. Now I think (and hope), you will be curious and ask why isn't my computer going far away from me then? The answer is - Gravity.
Gravity is the local gangster of the neighbourhood. Scientifically, it is much stronger locally and dominates the effects of the expansion, and that is why all your belongings are exactly where they are and why earth is exactly the same distance away from the sun as it was 100 years ago. The problem comes when things are too far away from each other in the (maybe infinite) space, this makes the local gangster loose the war and the expansion starts to dominate. but how does this expansion affects us? doesn't seem that scary? and it might not be, but the problem is - It is not alone!
(Accelerated) Expansion of the Space
The rate of cosmic expansion is not constant. The universe is not expanding at the same speed today as it was one year or one million years ago; instead, the expansion rate is dynamically changing. Crucially, the velocity of this expansion is actively accelerating, driven by an elusive force known as dark energy.
The Day the Stars Will Disappear
Now we finally reach the reason behind the title of this blog.
No, the stars are not going to suddenly switch off one random Tuesday. The real reason is much stranger.
The universe is expanding, and that expansion is accelerating. Space itself is stretching faster and faster because of something scientists call dark energy.
But this raises an obvious question:
If a galaxy moves farther away, why can its light not simply reach us later?
The answer is that the distance is not just increasing. The space between us and the galaxy is expanding faster and faster.
The Running Man Analogy
Imagine that you are running toward a finish line at a maximum speed of 10 mph.
If the running track stretches at 5 mph, you can still reach the finish line. It will simply take longer.
But if the track stretches at 15 mph, the finish line moves away faster than you can run. You are still running forward, but the distance between you and the finish line keeps increasing.
Light faces the same problem.
A particle of light, called a photon, always travels at the speed of light. However, if the space between us and a distant galaxy expands faster than the photon can cross it, the light will never reach us.
The photon does not slow down or stop. It keeps moving toward Earth forever, but the distance it must travel grows even faster.
The Cosmic Point of No Return
There is an invisible boundary around us called the cosmological event horizon.
When a galaxy crosses this boundary, any new light it emits will never be able to reach us. Its stars may continue shining, exploding and forming planets, but their future will become permanently invisible.
In roughly 100 billion years, almost every galaxy outside our Local Group, our neighbourhood of around 80 galaxies, will have crossed this horizon.
Their light will not suddenly disappear, though. We will still receive older light that began its journey before they crossed the boundary.
The Final Fade
As this old light travels through expanding space, its waves are stretched. This is called cosmological redshift.
The light first becomes redder, then moves into infrared and radio wavelengths. Over enormous periods of time, the waves become so stretched and faint that no telescope could detect them.
Over the next trillions of years, even the final traces of distant galaxies will fade from view.
The Loneliest Night Sky
By then, the Milky Way and Andromeda will have merged into one enormous galaxy. Any intelligent life living there may look into the sky and see only the stars of its own galactic neighbourhood.
Every other galaxy will appear to have vanished.
Not because they stopped existing.
Not because their stars stopped shining.
But because their light can no longer cross the ever-expanding space between us.
Future astronomers might have telescopes far more powerful than ours, yet they would still see nothing beyond their own galaxy. They might never discover that billions of other galaxies once existed. They might never find clear evidence of cosmic expansion or even the Big Bang.
To them, their galaxy might appear to be the entire universe.
That is what makes our time so special. We are alive while the ancient light of distant galaxies is still reaching us and while the universe is still revealing its history.
The stars are not really disappearing.
Their light is.
What If This Has Already Happened to Us?
And here is one final thought I had that might keep you awake tonight. What if our observable universe was once just like a galaxy in a much larger universe? Maybe countless other galaxies or regions existed beyond it, but over time, expansion carried them so far away that their light could no longer reach us and now we are left with this only galaxy, that we refer to as the "observable universe".
Now imagine intelligent life appearing trillions of years from today, after every distant galaxy has disappeared. They would see only the stars inside their own merged galaxy, with no clue that anything else had ever existed. Even if they somehow discovered the truth, they would still be trapped. Since nothing can travel faster than light, they could never catch the galaxies escaping beyond the cosmic horizon. They would be stuck inside their lonely island of stars forever.
But do not worry. You are not going to wake up tomorrow and find the night sky empty. This will happen hundreds of billions, and eventually trillions, of years from now. For the moment, the universe is still full of galaxies, their ancient light is still reaching us, and we are lucky enough to be here while the cosmos is still revealing its secrets.
Finally, I will ask you a question, after reading my thoughts-
<quote> With curiosity,Do you think the Universe is truly infinite?
Piyush Pant </quote>